1. Field of the Invention
The present invention relates to injection molding methods and apparatus. More particularly the invention relates to improved methods and apparatus for gas assisted injection molding which is used where it is desired to make hollow plastic parts for the purposes of saving weight and material as well as providing improved surface finishes.
2. Description of the Prior Art
In the infancy of fluid injection molding, articles were generally produced by injecting a molten plastic material into a mold cavity and letting the material cool to form a molded article, at which time the mold cavity was opened and the article was released.
However, as the art of plastic molding advanced to allow the manufacture of larger and more complicated parts, it became evident that in parts having thick rib sections, or complicated cross-sections, the surface finishes of the surfaces opposite such portions of the molded part would suffer from sink marks and the like. In addition, as the size of the molded parts which were able to be produced increased, the cost of the material and the weight of the parts increased substantially.
This led those skilled in the art to develop methods of making these large and complicated parts of a hollow construction by injecting a fluid or gas into the interior of the part while it was being formed in the mold cavity. It was discovered that as the gas or fluid was injected into the interior of the mold cavity it would follow the path of least resistance. Since it is well known that the molten plastic will generally cool and harden from the outside inwardly, the gas would penetrate the inner, softer and warmer portions of the part, which would normally be found in the thicker sections where there is a structural rib, etc. By following this path, there was created a hollow within the plastic, and it could be expanded by the addition of pressure outwardly to fill the mold cavity. It was found that if the gas pressure was held at an effectively constant level while the plastic part cooled, the surface finish of the plastic part would be greatly enhanced, i.e., by the elimination of sink marks, etc.
However, along with the idea of the injection of the gas or fluid, came the need to vent the gas to relieve the pressure within the part before the mold could be opened. Many solutions were tried to discover a satisfactory way of introducing the gas into the interior of the plastic part, and then to vent the gas before opening the mold cavity.
Several types of apparatus have been produced. In one type, only plastic molding material is introduced into the mold cavity through the injection nozzle, which is in contact with the mold sprue. In this apparatus, called "pin in runner", the venting of the gas takes place through the runner in the mold cavity. However, this normally leaves one or more holes in the hollow injected molded part when the "runner" is removed, which may be covered by a plug. Alternatively, complicated apparatus may be used for sealing the hole in the cavity wall through which the gas enters the cavity by injecting a further quantity of plastic material. However such attempts have generally not met with commercial acceptance, particularly for parts which require surface finish.
In a second type of apparatus, called "pin in article", it is known to inject both plastic and fluid or gas through the injection nozzle and the mold sprue, and then to mechanically vent the molded part by piercing or boring a vent opening in the wall of the molded article. However, in many cases, the act of piercing itself does not guaranty that gas will escape unless a hollow piercing tool is used. In some cases, even the use of a hollow tool has not successful because the molten plastic blocks the hollow tool during the step of introducing the plastic material into a mold space.
In another similar type construction, the mold cavity is shaped such that an easily removable portion of the wall of the molded material is formed by the provision of "break" lines, and this plug is pulled out of the mold to vent the part. However this method also results in a hole in the part which has to be plugged, which is unacceptable for many types of molded articles.
In another type of construction, it is known to introduce the gas through the injection nozzle which introduces the plastic material, and then to vent the cavity by a step involving creating a gap between the nozzle and the mold. This allows the gas to pass from the cavity to atmosphere. This step is known as "sprue break". However this method also has a number of disadvantages. Under present day circumstances, it is considered undesirable to vent the gas into the environment of the molding machine for health reasons. Further, some molding machines are not equipped to permit a "sprue break", and the apparatus needed to contain the gas which is vented is complicated and expensive. Thus this method also unsatisfactory for some applications, and an improved method which eliminates uncontrolled venting is needed.
For these reasons, many in the art have gone to a still further known arrangement wherein there is provided, in association with the supply of pressurized gas, a single injection nozzle having a first passage for injection of a thermo-plastic, and a second passage for the introduction of a pressurized gas. During the supply of pressurized gas through the nozzle to the mold cavity, a check valve is initially used to keep the thermo-plastic out of the second passage. The pressurized gas is then introduced into the mold cavity, after the flow of thermo-plastic starts. The flow of thermo-plastic is then completed, but the flow of the pressurized gas continues until a certain volume is introduced. The mold is then held under pressure until the molded part is sufficiently cooled, at which time, after venting, the mold can be opened. U.S. Pat. Nos. 4,781,554 and 4,855,094 of James W. Hendry are examples of this type of operation.
However, these and other apparatus, which have heretofore operated on the principle that a flow of molten material must first be started, and then a flow of pressurized fluid or gas is started, have suffered from the problem of clogging of the gas supply passages due to the molten material entering and clogging the gas supply passages during the initial injection of molten plastic. This involves costly down time and complicated apparatus to try and prevent the entry of the molten molding material into the gas supply system. It was not until the discovery that it is possible to sequentially first introduce a molding material into the mold cavity, and then shut off the supply of plastic, and thereafter introduce the pressurized gas through the injection nozzle into the interior of the mold cavity, that a satisfactory solution to the problems in the prior art was found.